Molded plastic septic tanks have been used commercially in substitution of older and more widely used concrete tanks. Prior art plastic tanks have been made from thermoplastic or thermoset plastic, as for instance is exemplified by fiberglass-resin tanks. Plastic septic tanks have tended to be generally rectangular or round in cross section. They are often heavily ribbed or corrugated for strength, when buried in soil. Typical popular volume capacities are in the range 1000-1500 gallons. Septic tanks must have top openings with closures, to permit maintenance. Most septic tanks have at least one baffle which limits lengthwise flow within the tank, and divides the tank into an input end portion and the output end portion.
Thermoplastic septic tanks have typically been made of polyethylene or other polyolefin, using processes which include blow molding and rotational molding. The walls of prior art polyethylene tanks have been made of fully dense, or solid, polymer, with thicknesses in the range of one quarter to three eights of an inch. Examples of such tanks can be seen in various U.S. patents.
Depending on a particular user's viewpoint, thermoplastic septic tanks offer advantages and disadvantages compared to concrete tanks. Advantages include light weight and thus easy portability, resistance to fracture from dropping or freezing, and corrosion resistance. Disadvantages compared to concrete tanks have included higher cost, lower strength under load, susceptibility to local impact damage, light weight (which makes them buoyant when in watery soil) and lower stiffness, which can result in distortion during installation and use.
Thermoplastic septic tanks have had modest market place acceptance in substitution of the concrete tanks. Many installers only favor them when their light weight eases access to sites having difficult terrain. Thermoplastic septic tanks have tended to be more costly than comparable volume concrete tanks. That can be attributed in large part to the amount of comparatively costly plastic material and labor of manufacture.
Prior art plastic septic tanks have evidenced other limitations in varying degree, which the present invention seeks to address. These include undue deformation prior to installation, during installation and during use. In particular, a tank might deform too much when it is vacuum tested prior to use; or it may deform due to pressure of the surrounding soil when it is emptied for maintenance. During installation, plastic tanks walls have been penetrated by materials handling equipment and the impact of stones. Tanks have experienced short term and long term deformation under static loads from water and soil, including loads applied by any buoyancy hold-downs. Thus there is a continued need for improvement in the foregoing and other respects.